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High arsenic abundance of 50–700 μg/L in the groundwater from the Chianan Plain in southwestern Taiwan is a well-known environmental hazard. The groundwater-associated sediments, however, have not been geochemically characterized, thus hindering a comprehensive understanding of arsenic cycling in this region. In this study, samples collected from a 250 m sediment core at the centre of the Chianan Plain were analyzed for arsenic and TOC concentrations (N = 158), constituent minerals (N = 25), major element abundances (N = 105), and sequential arsenic extraction (N = 23). The arsenic data show a prevalence of > 10 mg/kg with higher concentrations of 20–50 mg/kg concentrated at 60–80 and 195–210 m. Arsenic was extracted mainly as an adsorbate on clay minerals, as a co-precipitate in amorphous iron oxyhydroxide, and as a structural component in clay minerals. Since the sediments consist mainly of quartz, chlorite, and illite, the correlations between arsenic concentration and abundances of K 2 O and MgO pinpoint illite and chlorite as the major arsenic hosts. The arsenic–total iron correlation reflects the role of chlorite along with the contribution from amorphous iron oxyhydroxide as indicated by arsenic extraction data. Organic matter is not the dominant arsenic host for low TOC content, low arsenic abundance extracted from it, and a relatively low R 2 of the arsenic–TOC correlation. The major constituent minerals in the sediments are the same as those of the upriver metapelites, establishing a sink–source relationship. Composition data from two deep groundwater samples near the sediment core show Eh values and As(V)/As(III) ratios of reducing environments and high arsenic, K, Mg, and Fe contents necessary for deriving arsenic from sediments by desorption from clay and dissolution of iron oxyhydroxide. Therefore, groundwater arsenic was mainly derived from groundwater-associated sediments with limited contributions from other sources, such as mud volcanoes. [ABSTRACT FROM AUTHOR]

The whole-rock Hf, Sr and Nd isotope data of five high-Fe–Ti eclogites, nine high-Al eclogites, and a garnetite from the Sulu ultrahigh pressure (UHP) metamorphic terrane at eastern China were analyzed to resolve the causes for the decoupling of the Lu–Hf, Sm–Nd, and Rb–Sr isotope systems in these UHP rocks and to infer their protolith characteristics. Seven of the nine high-Al eclogites define an 87 Rb/ 86 Sr– 87 Sr/ 86 Sr errorchron age of 192 ± 43 Ma (MSWD = 2.8), which is within the time span of retrograde metamorphism despite the large uncertainty. The high-Fe–Ti eclogites and garnetite, however, have low 87 Rb/ 86 Sr ratios of < 0.031 with scattered 87 Sr/ 86 Sr ratios of 0.7042–0.7058. Accordingly, it is inferred that the Rb–Sr isotope system in the samples reflects the effects of processes postdating the UHP metamorphism. Despite having different constituent mineral assemblages and whole rock geochemistry, the samples, however, define a 147 Sm/ 144 Nd– 143 Nd/ 144 Nd errorchron age of 232 ± 36 Ma (MSWD = 4.6). Although the uncertainty of ± 36 Ma implies incomplete Nd isotope equilibrium among the samples, the large overlap between this errorchron age span and the mineral isochron ages of 245–210 Ma for the UHP metamorphism indicates the control of peak metamorphism on the Sm–Nd isotope system. The incomplete Nd isotope re-equilibration was accompanied by metamorphic modification on the Sm/Nd ratios as indicated by the U-shaped LREE patterns. The initial ε Nd (780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial ε Nd (t)–ε Hf (t) trend to significantly lower values, consistent with the metamorphic increase in the Sm/Nd ratios. In contrast, the Lu/Hf ratios are generally within the range for basalts and do not vary systematically with the 176 Hf/ 177 Hf ratios. The protolith ε Hf (780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial ε Hf (t) values of the ~ 780 Ma magmatic zircon cores from the Yangtze craton, suggesting the dominance of protolith characteristics on the Lu–Hf isotope system. Being controlled by different processes, the Rb–Sr, Sm–Nd, and Lu–Hf isotope systems of the investigated samples are therefore decoupled. The ε Nd (220)–ε Hf (220) compositions of the samples deviate from the ε Nd (t)–ε Hf (t) trend of intraplate lavas to high ε Hf (220) at a given ε Nd (220) and are within the arc lava field. The protolith ε Nd (780)–ε Hf (780) compositions calculated from metamorphic initials using igneous Sm/Nd and Lu/Hf ratios show a similar distribution pattern, strengthening the arc affinity of the eclogites. These arc signatures together with the felsic–mafic bimodal geochemical features of the UHP rocks from the Sulu terrane are explained as the characteristics of protoliths generated by backarc rifting. [ABSTRACT FROM AUTHOR]

Cenozoic (Miocene to Pleistocene) basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt. We present major, trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area. On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province, we suggest that these basalts may belong to the same magmatic system. The alkali basalts found in Jiangsu province have higher σFeO, MgO, CaO, Na2O, TiO2 and P2O5 and incompatible elements, but lower Al2O3 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine, pyroxene and minor plagioclase. In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/σFeO ratios. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB. Partial loss of the mantle Uthosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma. The 143Nd/144Nd vs. 87Sr/86Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EM1 component in the study area. According to Shaw's equation, the basalts from Jiangsu province may be formed by 1%-5% partial melting of a depleted asthenospheric mantle source. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen (1982), we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts. [ABSTRACT FROM AUTHOR]

Abstract: The widespread Mesozoic granitoids in South China (∼135,300km2) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n =7) and FZC (n =6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO2 content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI=0.97–1.08 and εNd(T)=−8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI>1.1 and εNd(T)<−11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI<1.0 and εNd(T)>−6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T)>−8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites. [Copyright &y& Elsevier]

Abstract: The present study deals with geochemical characterization of mud fluids and sediments collected from Kunshuiping (KSP), Liyushan (LYS), Wushanting (WST), Sinyangnyuhu (SYNH), Hsiaokunshui (HKS) and Yenshuikeng (YSK) mud volcanoes in southwestern Taiwan. Chemical constituents (cations, anions, trace elements, organic carbon, humic acid, and stable isotopes) in both fluids and mud were analyzed to investigate the geochemical processes and spatial variability among the mud volcanoes under consideration. Analytical results suggested that the anoxic mud volcanic fluids are highly saline, implying connate water as the probable source. The isotopic signature indicated that δ18O-rich fluids may be associated with silicate and carbonate mineral released through water–rock interaction, along with dehydration of clay minerals. Considerable amounts of arsenic in mud irrespective of fluid composition suggested possible release through biogeochemical processes in the subsurface environment. Sequential extraction of As from the mud indicated that As was mostly present in organic and sulphidic phases, and adsorbed on amorphous Mn oxyhydroxides. Volcanic mud and fluids are rich in organic matter (in terms of organic carbon), and the presence of humic acid in mud has implications for the binding of arsenic. Functional groups of humic acid also showed variable sources of organic matter among the mud volcanoes being examined. Because arsenate concentration in the mud fluids was found to be independent from geochemical factors, it was considered that organic matter may induce arsenic mobilization through an adsorption/desorption mechanism with humic substances under reducing conditions. Organic matter therefore plays a significant role in the mobility of arsenic in mud volcanoes. [Copyright &y& Elsevier]

Abstract: The Wuchagou and Halaha volcanic fields are located in the central Great Xing'an Range, Northeast China, cover an area of ~1400km2 in China, and continue westward into Mongolia. These regions provide an important opportunity to study temporal changes in magma source regions accompanying the evolution of geodynamic-processes. Late Miocene (10.5–8.0Ma) volcanic rocks from the Wuchagou volcanic field are composed of basaltic andesite and andesite. The characteristics of these rocks include a high concentration of Si (SiO2 =55.7–59.2wt.%), a pronounced negative Nb―Ta anomaly, and a slightly LREE enriched pattern with (La/Yb)N ratios varying from 2.1 to 6.9. They have Sr and Nd isotopic compositions indicative of long-term incompatible element enrichment (87Sr/86Sr=0.7053–0.7067, 143Nd/144Nd=0.5118 to 0.5122), with a trend toward the EM1 component. Their isotopic compositions can also be described as Dupal-like, but with less-radiogenic Pb isotopic ratios (206Pb/204Pb=16.99–17.83, 207Pb/204Pb=15.46–15.52, 208Pb/204Pb=37.42–37.91). Quaternary (2.0Ma to Holocene) lavas from the Halaha volcanic field consist of alkali olivine basalt and olivine tholeiite, which show alkalic affinities and are characterised by ocean island basalt-like REE and trace element patterns, typical of an intraplate sodic basalt composition. The low 87Sr/86Sr (0.7034–0.7041), high 143Nd/144Nd (0.5128 to 0.5130), and moderately radiogenic 206Pb/204Pb (18.39–18.55), 207Pb/204Pb (15.54–5.55), and 208Pb/204Pb (38.32–38.47) ratios are similar to those of most late Cenozoic basalts of South China and the Abaga region, Inner Mongolia. The different melt compositions between the volcanic rocks of the two volcanic fields are most likely due to changes in geodynamic processes and magma sources. A possible scenario for the genesis of these volcanic rocks is that Wuchagou andesitic magma formed by high degrees of partial melting of the subcontinental lithospheric mantle with some lower-crust contamination as a result of the thermal reactivation of the continental lithosphere by shallow-level rifting following the cessation of the Japan Sea opening. During the Quaternary, extensive volcanism resumed, with eruptions along deep rifting systems in the Halaha volcanic field, which might have been related to the geodynamic forcing of the continuous piling up and upward thickening of the stagnant subducted Pacific slab beneath Northeast China. Basaltic magmas of this stage were primarily produced by melting of convecting asthenosphere. Using the batch melting of a hypothetical LREE-enriched mantle source in the garnet stability field proposed by Xu et al. (2005), we estimated that the Halaha basalts could be generated from 9 to 14% of partial melting of garnet peridotites. [Copyright &y& Elsevier]

Abstract: In this study, the geochemical characteristics of mud volcanoes were investigated to seek their possible linkage with arsenic mobilization in the region. Among the mud volcanoes of southern Taiwan, the largest and well-preserved active mud volcanoes are situated in Wushanting (WST) and Hsiaokunshui (HKS). These two selected active mud volcanoes are exhaling mainly methane (CH4 >95%) along with other gases like O2, CO2, CO, SO2, H2S, NO, and N2. The content of CH4 in the HKS active mud volcano is very close to that of the WST mud volcano. High Na+ and Cl− concentrations in mud liquids and mudstone deposits indicate their origin in a marine depositional source. However, a newly erupted active mud volcano in WST reflects the presence of a high level of SO4 2−. Significant variations (p ≤0.05) in trace and transition element (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Cd and Ba) concentrations in the liquids and sediments of mud volcanoes are noticed at different eruption periods, suggesting different elemental compositions and release mechanisms in different locations. The weathering of micas as well as clay minerals seems to be responsible for the enrichment in trace metals in mud volcano sediments and fluids. Several trace elements (V, Cr, Mn, Fe, Co, Ni, and Zn) are strongly bound to the sediments, but concentrations of Cu, Se, and Sr are significantly higher in the fluids. The presence of higher As contents in the sediment of HKS was observed in contrast to its fluid. As a reducing agent, humic substances seem to play an important role in binding with trace elements and particularly As in the sediments. Fourier Transform Infrared (FTIR) spectra of humic substances from muds showed the presence of possible functional groups of primary and secondary amines, ureas, urethanes (amide) and silica. [Copyright &y& Elsevier]

Abstract: The southeastern Eurasian continental margin has been characterized by formation of rift basins associated with intraplate basaltic volcanism since early Cenozoic time. In contrast to Paleogene volcanic rocks that occur sporadically in the basins, Neogene basalts are more widespread on land as lava flows and pyroclastics in the Taiwan Strait (Penghu Islands) and northwestern Taiwan. To better understand the tectonomagmatic evolution, in particular the initial rifting record, this study reports new age, major- and trace-elemental, and Sr-Nd-Pb isotope data of volcanic rocks drilled from several locations in the Taiwan Strait and western Taiwan. 40Ar/39Ar dating results show two main episodes of volcanic activities: ~56-38Ma (Eocene) and ~11-8Ma (late Miocene). The volcanic rocks are composed dominantly of basalts and basaltic andesites, and subordinately of dacites and rhyolites of Eocene age. The two episodes of basaltic volcanism have distinct geochemical characteristics. Comparatively, the Eocene basalts are more depleted in basaltic components such as Ca, Fe and Ti, but have higher Al content. They are also more enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and show depletions in high field strength elements (HFSE). Sr-Nd-Pb isotope compositions of the late Miocene basalts are relatively more uniform and unradiogenic (εNd=+6.0 to +3.8), similar to those of Miocene basalts from NW Taiwan and Penghu Islands, and broadly coeval OIB-type basalts from the South China Sea. However, the Eocene basalts have a wider range in isotope ratios (e.g., εNd(T)=+5.6 to -3.2) pointing towards an enriched mantle source. The overall geochemical characteristics suggest two distinct mantle sources: (1) a more refractory mantle source metasomatized by subduction-related processes to generate the Eocene basalts and (2) a fertile but isotopically depleted mantle source for the late Miocene basalts. These two source components are proposed to reside in the lithospheric mantle and asthenosphere, respectively. The change in magma sources with time reflects the evolution of an extensional regime within the Eurasian continental margin from an initial rifting to a well-established stage accomplished by thinning of the lithosphere and associated upwelling of the asthenosphere. The Eocene bimodal volcanism entails a transition from the latest Cretaceous magmatism in the western Taiwan Strait that not only signals incipient rifting in the region, but also records geochemical inputs from the subducted Paleo-Pacific plate to the southeastern Eurasian lithospheric mantle. As the preexisting, subduction-related component had been preferentially overprinted by the Eocene magma generation, there was a magmatic quiescence in the Oligocene before the onset of Miocene basaltic volcanism that resulted essentially from decompression melting of the ascended asthenospheric mantle. [Copyright &y& Elsevier]

Abstract: Permian volcanic rocks, including picrites, flood basalts and silicic volcanic rocks, crop out in the Song Da district, northern Vietnam. They occur as a ∼300-km-long belt emanating from the southernmost part of the Emeishan large igneous province, SW China. The belt represents a detached part from the province and was displaced ∼600km southeastward along the Ailao Shan-Red River shear zone during the mid-Tertiary in response to the India–Eurasia collision. We report here geochemical and Sr–Nd isotopic data on the Song Da volcanic rocks to decipher the petrogenetic processes responsible for their formation. The picrites show evidence of olivine fractionation and formed from high-Mg magmas (MgO=20–25wt.%) originated from depleted mantle peridotites from the Emeishan plume head. The plume head is characterized by εNd(t) value of +8. Some picrites and low-Ti basalts have low Nb/La, Nb/U and εNd(t), which are indicative of plume–lithosphere interaction in various degrees. Contamination of continental crustal rocks (<20%) is able to produce the spectra of the observed trace element patterns and isotopic ratios. The silicic volcanic rocks are metaluminous or peraluminous (aluminium saturation index=0.92–1.24) and are sufficiently different from those present elsewhere in the Emeishan large igneous province that are dominantly peralkaline. The Song Da silicic volcanic rocks have εNd(t) values of −0.1 to +0.6 and show strong depletion in compatible trace elements (e.g. Ni=0.8–3ppm, V=4–55ppm). They most likely formed by extensive fractional crystallization from the associated high-Ti basalts, possibly with some crustal contamination. [Copyright &y& Elsevier]

Abstract: Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5m) and piezometer wells (depth 12.2–79.2m) demonstrate that the groundwater is mostly the Ca-HCO3 type and anoxic in nature (mean EhSHE =34mV). Arsenic concentrations ranged from <10–538μg/L, with high concentrations only present in the shallow to medium depth (30–50m) of the aquifer along with high Fe (0.07–9.8mg/L) and relatively low Mn (0.15–3.38mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO3, SO4 and NO2 along with higher values of DOC and HCO3, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain. [ABSTRACT FROM AUTHOR]

Abstract: Core sediments from three boreholes (to a depth of 50m) and groundwater from 20 As-enriched water wells were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analyses. Multivariate statistical analyses including factor analysis, cluster analysis and multidimensional scaling were applied to the hydrogeochemical data. The results show that a few factors adequately represent the traits that define water chemistry. Sodium, Cl, SO4, total dissolved solids (TDS), and electrical conductivity (EC) are grouped under the first factor representing the salinity sources of waters. The second factor, represented by As and Mn, is related to As mobilization processes. The third factor of Fe and alkalinity is strongly influenced by bacterial Fe(III) reduction which would raise both Fe and concentrations in water. The fourth factor of Ca and Mg reflects the hardness of the Ca–HCO3 type of groundwater, which is confirmed by the hydrochemical facies analysis. Cluster analysis leads to the formulation of four water types including highly, moderately, and slightly As-enriched groundwater as well as groundwater with elevated , from anthropogenic sources. Multivariate analyses of the geochemical parameters suggest that Fe- and Mn-oxyhydroxides and mineral phases of phyllosilicates (e.g., biotite) are the main hosts of As in the sediments. Statistical analysis also shows that As is closely associated with Fe and Mn in sediments while As is positively correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing As into the groundwater. [Copyright &y& Elsevier]

Abstract: Major ion, trace element, and stable isotope analyses were performed on groundwater samples collected during November 2005 and 2006 in Chia-Nan plain of southwestern Taiwan to examine As mobilization in aquifers. The high concentrations of As, Fe and Mn in the groundwater is consistent with low Eh values (under moderately reduced state). Moreover, the observed Na/Cl and SO4/Cl molar ratios in groundwater demonstrate the influence of seawater intrusion. Seawater intrusion could provide required electron acceptors (i.e., SO4) for bacterial sulfate reduction and promote reducing conditions that are favorable for As mobilization. The concurrent increases in the concentrations of Fe and Mn from 2005 to 2006 may be caused by bacterial Fe(III) and Mn(IV) reduction. Geochemical modeling demonstrate that As(III) is the dominant As species and the presence of Fe-bearing carbonates, sulfides, and oxide phases may locally act as potential sinks for As. Mud volcano fluids were also collected and analyzed to assess the possible source of As in the Chia-Nan plain groundwater. The oxygen and hydrogen isotopic signatures indicate that the As-rich mud volcano fluids may have been modified by chemical exchange with 18O-rich crustal rocks and possibly originated from mixing of deep brines with circulating meteoric water. Thus As in the Chia-Nan plain groundwater may have been evolved from deep crustal fluids or rock sources. The hydrogeochemistry and widespread As enrichment in groundwater of Chia-Nan plain result from multiple processes, e.g., de-watering of deep crustal fluids, seawater intrusion, and biogeochemical cycling of Fe, As, and S in alluvial sediments. [Copyright &y& Elsevier]